System, method and apparatus for fluidized bed additive manufacturing

ABSTRACT

A system, method and apparatus for additive manufacturing is disclosed. The method includes fluidizing particles with a medium to form a fluidized bed and additively manufacturing an article formed from the particles. The article has an open porous structure defining a plurality of pores and a plurality of fluid paths through the article. The method further includes flowing the particles and the medium through the fluid paths while the fluid paths are being formed. The article may be additively manufactured by selectively sintering the particles at target areas on the article which are near the surface of the fluidized bed.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.16/353,939, filed Mar. 14, 2019, which claims the benefit of U.S.Provisional Application No. 62/643,632, filed on Mar. 15, 2018. Thecontents of the above-referenced applications are incorporated herein byreferences in their entirety.

FIELD OF THE DISCLOSURE

The present disclosure generally relates to additive manufacturing and,in particular, to a system and a method that uses a selectively directedenergy beam to sinter powder in a pattern to fabricate an article.

BACKGROUND OF THE DISCLOSURE

Additive Manufacturing has enormous potential for improving the wayarticles are made. Of the different additive manufacturing productionmethods available, laser sintering of powder is considered as one of themost promising of these methods. Laser sintering is a process based ondispensing powder from a hopper, spreading the powder in a smooth layerusing a roller and then selectively irradiating the powdered material.As a result of the irradiation, the polymer particle partially melts ormelts the surface of the particle. The particles then sinter by adheringto each other coalescing and solidifying thus producing the desiredshape of the layer of powder irradiated. The pattern of sintered powderin each layer corresponds to a slice of the article being fabricated.This layer-by-layer process continues by sintering each subsequentlyapplied layer. Repetition of the above steps results in the formation ofa laser-sintered article lying in a bed of unused powder. The completedarticle is then dug out of the powder bed and dusted off. In this waythe process can produce complicated three dimensional articles and issuitable for low volume production of high value articles.

Although the laser sintering technology described above is well suitedfor making low volume, high value articles with a complex shape, thereare a number of limitations that prevents this technology from beingsuitable for high volume production of articles:

-   -   The types of powder are limited to those with good flowability        such that the roller can lay down a smooth layer. Generally only        a few formulations such as, polyamide 11 and polyamide 12, are        available. Suitable powder formulations are not only very        limited, but also more expensive by at least an order of        magnitude than commodity plastics. A larger range of cheaper        powders is needed.    -   The amount by which the powder can be preheated is limited by a        caking temperature above which the powder starts to clump and        the roller cannot lay down a smooth layer.    -   Fabrication speed is an issue and is limited by including a step        to lay down powder with the roller.    -   Good temperature control of the most recent layers fabricated is        difficult in a quiescent powder bed. Poor temperature control        can lead to deformation of the article or curling up of layers        at edges and corners.    -   Heat removal from the fabrication process is inefficient in a        quiescent powder bed. The amount of heat supplied by the laser        or lasers can limit the speed of fabrication.    -   It is difficult to fabricate articles with a gradient in        composition.    -   It is cumbersome to remove the completed article from the bed of        unused powder.

SUMMARY OF THE DISCLOSURE

In general terms, the present disclosure can include a system, method,and apparatus that comprises fluidizing a bed of powder and using adirected energy beam to join (e.g., sinter) the particles of the powderin selected target areas. The target areas can be at or near the surfaceof a structure being fabricated. The pattern of the target areas cancorrespond to a cross-sectional slice of the model of the article beingfabricated. Sintering particles in a pre-determined pattern fabricatesportions of the structure being fabricated. Embodiments of the structureare designed to have an open porous structure. The porosity of thestructure is sufficiently open so as to allow movement of the particlesof the fluidized bed through the pores and to the build surface.

Embodiments can use the principle of powder fluidization to depositpowder at the build surface of a structure being fabricated by additivemanufacturing. Fluidization is a process whereby a granular powdermaterial is converted from a solid-like state to a fluid-like state.Fluidization can be achieved by passing a fluid (e.g., gas, liquid orsupercritical fluid) up through a powder bed. This is generally done ina vessel with a distributor plate. The powder is above the distributorplate and the fluidizing medium is forced up through the distributorplate and through the powder bed. For the powder to be fluidized, thefluid velocity is high enough that the drag forces on the particles ofthe powder overcome gravitational forces, causing the particles tobecome suspended and collide with each other. If the particles are smallenough, and the velocity is not so high as to form bubbles, then the bedexpands smoothly in a homogeneous manner, with the top surface beingwell defined. When a powder bed of solid particles is thus fluidized itexhibits liquid-like behavior. For example a fluidized bed of powder canfill the volume of a chamber like a liquid so that the surface of thefluidized bed is more or less flat and perpendicular to gravity. Objectscan be immersed in a fluidized bed. The liquid-like behavior of afluidized bed causes objects with higher density than the fluidized beddensity to sink. Also, fluidized powder can be transported, for examplechanneled through pipes. Further, the height of the surface of twoconnected fluidized bed vessels will tend to equalize. A wide variety ofpowder types are suitable for fluidization. The particles of the powdercan have a wide range of compositions, different shapes, sizes and sizedistributions. Fluidized beds are also known to provide excellent heattransfer.

In current laser sintering machines, the powder is dispensedlayer-by-layer by a roller to form a quiescent powder bed on top of thebuild surface of the article being fabricated. In contrast, embodimentsof this disclosure can supply powder to the build surface(s) throughpores in the open porous structure of the article being fabricated.Therefore, in general terms, this can be an “inside-out” or“exoskeleton” method for building up an article by using buildingmaterial supplied from or through the inside of the article. The articlebeing fabricated can be started on a build substrate that is held inplace in the fluidized bed. In some versions, the build substrate ismore or less horizontal and is porous with holes over at least theregion where the fabrication of the base of the article is to start. Theholes can be large enough and sufficient in distribution that thefluidized bed can pass through the holes of the substrate. At the startof the fabrication the substrate can be immersed slightly below thesurface of the fluidized bed and selectively irradiated with a directedenergy beam, such as a laser, so as to sinter the particles of thefluidized bed to the surface of the substrate in selected target areas.The target areas where the laser scans can be determined from a slicetaken of a three-dimensional model of the structure in the computer orfrom a mathematical description of the structure of the article. Thereare several embodiments of the method to fabricate the porous structureusing a fluidized bed to dispense powder. In one embodiment, as thestructure is built up with sintered particles, it is submerged into thefluidized bed so that recently built portions of the structure areslightly below the surface of the fluidized bed. In another embodiment,the build substrate or build surface is raised above the fluidized bed,selectively irradiated so as to become tacky or molten at the targetareas on the build surface and then submerged into the fluidized bed soas to pick up fresh particles. As well as lowering the structure beingfabricated into the fluidized bed, the fluidized bed can be raised byvarious means. The type of structure fabricated can be sufficiently openand porous so as to allow the fluidizing medium and particles of thefluidized powder to be transported through the structure beingfabricated. Because of this open porosity property, fresh powder cancontinue to be transported to the build surface by the fluidizing mediumthat flows through the porous structure.

A computerized model of the article can be used to direct the energybeam selectively to the target areas of the build surface correspondingto the cross section of the structure being fabricated. For example theboundary of the target area can be made up of scans of the laser focalpoint and the middle filled in by a raster scan. Alternatively a singlepoint of the laser focal point can be used for the build surfacecorresponding to the cross-section of a structural element such as astrut. A computerized means can be used to control the height of thefluidized bed surface and the height of the structure relative to thefluidized bed vessel. A computer can control operations includingfluidizing the bed, circulating the fluidizing medium, supplying powderto the fluidized bed and controlling temperatures, temperatures andflows.

Fluidized beds are excellent mediums for heat transfer, and thisproperty can be used to control the temperature of the powder beingbrought to the build surface and to moderate the temperature of freshlybuilt portions of the structure. The temperature of the fluidized bedcan be controlled so that the powder is close to but below thetemperature at which particles become tacky and stick to each other. Bycontrolling the temperature, less energy needs to be supplied to thebuild surface to heat the particles. By being immersed in the fluidizedbed, temperature gradients in the freshly built structure can bemoderated, which can help to prevent geometric distortion of thestructure. The upward flow of the fluidizing medium efficiently removesheat supplied by the energy beam to the build surface during thesintering process. The heated fluidizing medium can be removed from thetop of the fluidization vessel and then can be cooled by a heatexchanger. The cooled fluidization medium stream can then be circulatedback through the bottom of the fluidized bed vessel by a pump or acompressor. With efficient heat removal and good control of thetemperature of the particles and structure being fabricated and thevessel as a whole, faster build times than existing solutions arepossible.

During the sintering process, volatile by-products can be produced. Thevolatiles thus produced are carried away by the fluidizing medium, outof the fluidization vessel and to other areas of the process where aportion of the impurities can be removed by a separation unit. After thepurification step, the fluidizing medium can be circulated back to thefluidized bed.

This disclosure is not limited to a particular type of powder, but isadaptable to many powder types that are fluidizable. These materials caninclude plastic, metal, polymer, ceramic powders, powders of compositeparticles or a mixture of such powders. Powders with a wide range ofparticle shapes, sizes and size distributions can be fluidized and used.

The composition of the powder in the fluidized bed can be varied at aspecific rate by removing powder and adding powder of a different typeto the fluidized bed and thus changing the composition of the fluidizedbed at a specific rate. By doing so, the composition of the articlebeing fabricated can be varied with a specific gradient.

The porosity of the structure can take many forms and can be designed soas to allow the easy, generally upward flow of the fluidizing medium andfluidized particles through the pores. All of the pores can beinterconnected and can have regular or irregular structure. When theporous structure is immersed in the fluidized bed, the liquid-likeproperties of the fluidized bed allows it to fill the pores. Walleffects from the pore walls may tend to hinder fluidization, but theseeffects can be reduced by using larger pore sizes or changing the poretype, orientation or shape. For example, a porous structure with minimalwalls can be composed of a three dimensional honeycomb network of strutsinterconnected at nodes. Each strut can be fabricated by the method ofselective irradiation and particles sintering to the build surface.During fabrication of such a structure, the target areas of the buildsurface represent horizontal slices of a strut or node. The open porousstructure can be designed to impart specific properties to thefabricated article. For example, the method can manufacture article withan open porous structure that is light weight and has a highstiffness-to-weight ratio.

In a conventional laser sintering process where powder is dispensed byroller, the powder is sintered together layer-by-layer until thecompleted part is formed. In contrast, embodiments of the disclosure canutilize both layer-by-layer and continuous methods. In the continuousmethod, portions of the structure can be built up continuously whenparticles from the fluidized bed are allowed to continuously deposit atbuild surfaces irradiated by the directed energy beam.

In some embodiments, the fabrication process of the porous structure canbe continued so as to describe the volume of the intended article. Whenthe article is completed, the article can be raised out of the fluidizedbed by a lift mechanism, and/or the fluidized bed can be partiallydrained to a holding vessel so that the fluidized bed surface is belowthe base of the article or the build substrate. A number of usefulprocesses can be performed at the end of the fabrication process,including heating and quenching at specific rates or reacting with areactive gas. The vessel can then be cooled, isolated, purged and openedand the article can be removed from the vessel through a hatch.

The cost per part manufactured by the system and method above can bereduced by increasing the number of fluidized bed additive manufacturingunits at a factory. In an embodiment, much of the equipment associatedwith recycling, heating, cooling, cleaning and pressurizing thefluidizing medium and much of the equipment associated with heating andsupplying the powder to the fluidized bed is shared by an array of alarge number of fluidized bed additive manufacturing units.

As can be appreciated from the above general description, the method andsystem reduces many of the limitations associated with fabrication ofarticles by methods that use a roller. The “inside out” method offeeding the fresh powder to the build surface has several advantagesover the method of depositing powder layer-by-layer with a roller.Various aspects of my fluidized bed additive manufacturing system andmethod may have one or more of the following advantages:

-   -   A broad range of particle types, compositions, shapes and size        distributions can be used.    -   The efficient heat transfer in a fluidized bed allows the        temperature of the fluidized powder to be controlled and        temperature gradients in the fabricated structure to be        moderated. This can help reduce curling and geometric        distortion.    -   The upward flow of the fluidizing medium carries away heat from        the sintering process and out of the fluidizing vessel and can        help increase build speed when they would otherwise be limited        by cooling.    -   Powder can be transported to the build surface, without the use        of a roller. Eliminating the roller step means that build times        are potentially reduced.    -   Articles with specific gradient properties in their structure        can be produced by changing the composition of the powder in the        fluidized bed at a specific rate.    -   After the fabrication is completed, the article does not need to        be dug out of a static powder bed. The unused powder can be        removed by lowering the height of the fluidized bed surface        below that of the base of the article. This can help reduce        turnover times.    -   If a gas is used as the fluidization medium, the ease of        fluidization or heat removal can be improved, by pressurizing        the gas and so increasing the gas density and heat capacity. A        supercritical fluid such as supercritical CO₂ can be used. Metal        powders have high density and that can make them difficult to        fluidize smoothly. For this reason a liquid or supercritical        fluid may be useful as fluidizing medium for metal powders.    -   The upward flow of the fluidizing medium helps to remove        volatiles that can be produced during the sintering process.    -   Equipment can be shared by multiple fluidized bed additive        manufacturing units in an array of fluidized beds.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate some of the embodiments of the system, methodand apparatus.

FIG. 1 depicts a schematic view of an example of the fluidized bedadditive manufacturing system.

FIG. 2 a-2 d depict views of various stages of fabrication of an exampleof a porous structure on a porous support.

FIG. 3 a-3 g depict perspective views of various stages of fabricationof an example of a porous structure on a porous support.

FIG. 4 depicts a schematic view of an example of an array of fluidizedbed additive manufacturing units.

FIG. 5 depicts one or more of the many structures and/or arrangementsthat may be created by the method.

FIG. 6 a-f depicts of additional structures that may be created by themethod.

FIG. 7 a-e depicts additional structures that may be created by themethod.

DETAILED DESCRIPTION

Embodiments of a system and a method of forming an article having aporous three dimensional structure using additive manufacturing aredisclosed. The system can use selective laser scanning technology tosinter powder at a build surface. The powder is admitted to the buildsurface by a fluidized bed. In some versions, the powder at the buildsurface is selectively sintered to fabricate a portion of a porousstructure. The porous structure thus fabricated is the core of thearticle being built. A skin can be attached to the porous core structureto make the surface continuous instead of porous.

FIG. 1 provides a schematic view of an embodiment of the fluidized bedadditive manufacturing (FBAM) system 10. The system 10 comprises aprimary vessel 12 that is impermeable fluids and may be insulated. Theprimary vessel 12 has a top and bottom being vertically spaced and mayhave a window 104 at the top. The primary vessel 12 defines a chamberand may have hatch 98 which can be opened to provide access to thechamber for inserting and removing articles 96 and cleaning. The hatch98 is closed during manufacturing. A distributor plate 14 is disposednear the bottom of the primary vessel 12 and may be attached to theprimary vessel 12. The distributor plate 14 extends horizontally acrossthe chamber and has a plurality of holes which may be distributed evenlyacross the distributor plate 14. The distributor plate 14 can be aperforated plate or a porous diffusor or other suitable choices that areavailable.

Embodiments of a fluidized bed 16 can be disposed in the chamber abovethe distributor plate 14 and extends vertically from the distributorplate 14 to a surface 18. The fluidized bed 16 is dynamic and maycontain peaks and troughs. Herein, the surface 18 is referring to ahorizontal line below which something would be submerged in thefluidized bed 16. The fluidized bed 16 can include particles and amedium having a superficial velocity u that is greater than the minimumfluidizing superficial velocity u_(mf) for the particles but less than aminimum bubbling velocity. In some examples, the superficial velocitymay be calculated by dividing the volumetric flow of the medium throughthe primary vessel by the cross sectional area of the primary vessel 12.The particles have an average outer dimension d_(p) of at least about 10microns and not greater than about 1 mm. In one embodiment, thefluidized bed may have a transparent zone that is at least partiallytransparent and extends a distance D below the surface 18.

In some embodiments, a lift device 90 is in the chamber and is attachedto the primary vessel 12. The lift device 90 may vertically move a frame92 attached to the lift device 90 which extends at least partiallyaround the perimeter of the primary vessel 12 and has at least oneopening 93 defined by a ledge. A substrate 94 extends across the opening93 and is received and secured by the ledge. The substrate 94 definesperforations which are larger than d_(p) such that particles can flowthrough the perforations and may be of varying size.

Embodiments can include a first camera 106 mounted adjacent to theprimary vessel 12 and pointed toward the substrate 94 for visuallymonitoring fabrication of the article. A second camera 108 of theinfrared type may be mounted adjacent to the primary vessel 12 fordetecting temperature variations within the chamber. The cameras 106,108 monitor the level of the fluidized bed 16 and the state andtemperature of the build. The output of the cameras is used by acomputer 28 to determine the height of the surface of the fluidized bed16. The cameras 106, 108 can also be used to determine the height of thearticle 96 above the substrate 94 when it is lifted above the surface 18of the fluidized bed 16. The first camera 106 can be used to monitor fordefects in the fabrication and the second camera 108 can be used todetect temperature variations. It can be appreciated that more than twocameras and cameras of different type and resolution can be used.

Some embodiments of the system 10 can comprise an energy beam source 100which emits an energy beam 101 (shown by a dashed line) toward opticalcomponents 102. The energy beam 101 can be a laser or an electron gun.The beam 101 can be manipulated by means of optical components 102. Itcan be appreciated that there are many means to manipulate laser beams101. Beam 101 can be focused, split into multiple beams, pulsed,selectively directed in different directions, selectively masked and/orotherwise manipulated by the optical components 102. Optical components102 comprise a prism or prisms and a mirror assembly for selecting thedirection of travel of the laser beam or beams 103. It can beappreciated that there are many means of directing the aim of a laserbeam, here the term aim meaning the propagation direction of the beam.The mirror assembly comprises mirrors and galvanometers coupled to themirrors to selectively orient them. The movement of the galvanometers iscontrolled by the computer 28 so that the aim of the laser beam can bedirected to scan in the target area on or near the surface 18 offluidized bed 16 according to different patterns determined by across-section of a model of the article 96 to be fabricated. The laserbeam or beams 103 may enter the fluidized bed vessel through window 104.It can be appreciated that in different embodiments, some of the opticalcomponents 102 can be placed inside primary vessel 12. Computer 28controls the optical components 102 so as to direct the energy beam 101at selective parts of a target at or near surface 18. The energy beam101 may be split into a plurality of energy beams 103 and the pluralityof energy beams 103 are directed through the window 104 into the chamberand toward the substrate 94.

Embodiments of a primary resistance heater 56 may be coupled to theprimary vessel 12 and may extend into the chamber above the distributorplate 14 for regulating the temperature of the chamber and the fluidizedbed 16. A first temperature monitoring device 54, which may athermocouple, thermowell or any suitable temperature monitoring device,is disposed in the chamber for monitoring the temperature of thefluidized bed 16.

In some examples, a first pressure tap 38 may be coupled to the primaryvessel 12 toward the top and above the fluidized bed 16 and a secondpressure tap 39 may be coupled to the primary vessel 12 directly abovethe distributor plate 14 for monitoring the pressure and calculating themass of particles in the fluidized bed 16 based on the difference inpressure between the pressure taps 38, 39.

Versions of the system 10 may also comprise a holding vessel 30containing the medium. The holding vessel 30 may be fluidly connectedthrough a first line 32 to a first valve 34 coupled to the first line 32that controls flow from the holding vessel 30 and may be fluidlyconnected to a first flow meter 36 which monitors the flow of the mediumfrom the holding vessel 30 through the first line 32 such that themedium can flow between the holding vessel 30 and the chamber.

Embodiments of the system 10 may also comprise a second line 23 which isfluidly connected to the first line 32 and to a second flow meter 26attached to the second line 23 for monitoring the flow into the chamberand which is coupled to a second valve 24. The second valve 24 iscoupled to the second line 23 which controls flow into the chamber.

In one example, the system 10 may also comprise a third line 40 which isfluidly connected to the chamber and to a third valve 41 coupled to thethird line 40 for controlling flow of the medium and the particles outof the chamber. The third line 40 may be coupled to a third flow meterfor monitoring flow out of the chamber. The third line 40 may also becoupled to a unit 42 for separating particles from the medium.

In some versions, the system 10 may also comprise a recycle line 43coupled to a first cooling device 50 for cooling the medium in therecycle line 43. The cooling device may be a heat exchanger. The recycleline 43 may also be coupled to a second heat exchanger 52 for heatingthe medium in recycle line 43 and may be coupled a recycle-valve 53 forcontrolling flow out of the recycle line 43. The recycle line 43 mayalso be fluidly connected to a fourth line 45 for adding medium to therecycle line 43.

Other versions of the system 10 may also comprise a fifth line 48coupled to a fourth valve 49 and to a reservoir of purge fluid used topurge the primary vessel 12 when the fourth valve 49 and the secondvalve 24 are open and the recycle-valve 53 valve is closed.

Another example of the system 10 may also comprise a sixth line 22 forreceiving the medium from the fifth line 48 and the recycle line 43. Thesixth line 22 may be fluidly connected to a pump or compressor 20.

Still another embodiment of the system 10 may also comprise a seventhline 44 which may receive flow from the third line 40 for purificationin a separation unit followed by releasing the medium to the atmosphere.The seventh line 44 may be coupled to a seventh valve 46 for controllingflow through the seventh line 44 and to a seventh flow meter 47.

Versions of the system 10 may also comprise a salvage vessel 70containing additives and particles and the medium. The particles in thesalvage vessel 70 may be fluidized. The salvage vessel 70 may be fluidlyconnected to an eighth line 77, to an eighth valve 76 coupled to theeighth line 77, to a ninth line 61, to a ninth valve 62 coupled to theninth line 62, and to the chamber such that particles and the medium canbe conveyed from the salvage vessel 70 through the eighth line 77, theeighth valve 76, the ninth line 61, and the ninth valve 62 to thechamber. The system 10 may also comprise a fourth heat exchanger 78coupled to the salvage vessel 70 for heating particles and the mediuminside the salvage vessel 70. A second temperature monitoring device 79may be coupled to and extending into the salvage vessel 70 formonitoring the temperature inside the salvage vessel 70.

Examples of the system 10 may also comprise a storage vessel 60containing some of additives and some the particles. The storage vessel60 may be fluidly connected to a tenth line 63, a tenth valve 64 coupledto the tenth line 63, the ninth line 61, the ninth valve 62 and thechamber such that particles and the medium may be conveyed from thestorage vessel 60 through the tenth line 63, the tenth valve 64, theninth line 61, and the ninth valve 62 to the chamber. The system 10 mayalso comprise a storage heating device 68 attached to and extending intothe storage vessel 60 for heating particles inside the storage vessel60. The system 10 may also comprise a third temperature monitoringdevice 69 coupled to the storage vessel 60 for monitoring thetemperature inside the storage vessel 60.

Other versions of the system 10 may also comprise an eleventh line 71which is fluidly connected to the chamber and coupled to an eleventhvalve 72 and a twelfth valve 74 of the three-way type. The twelfth valve74 may be coupled to a twelfth line 73 which may be fluidly connected tothe salvage vessel 70 such that medium and particles may be conveyedfrom the chamber, through the eleventh line 71, the eleventh valve 72,the twelfth valve 74 and the twelfth line 73 to the salvage vessel 70.

Embodiments of the system 10 may also comprise a thirteenth line 75coupled to a twelfth valve 74 which may be fluidly connected to thestorage vessel 60 such that the medium and some of the particles may beconveyed from the chamber through twelfth valve 74 and the thirteenthline 75 to the storage vessel 60.

In one example, the system 10 may also comprise a fourteenth line 67fluidly connected to the storage vessel 60 for conveying particles tothe storage vessel 60.

Versions of the aforementioned system 10 may be used to perform themethod of fluidized bed additive manufacturing. The particles in theprimary vessel 12 may be fluidized by means of a medium (liquid, gas orsupercritical fluid) that passes up through the distributor plate 14.Pump or compressor 20 forces the fluidization medium through lines 22,23 into the base of the primary vessel 12. The flow of the medium intothe primary vessel 12 may be controlled by opening and closing thesecond valve 24. The second flow meter 26 monitors the flow through thesecond line 23. The second valve 24 and second flow meter 26 areconnected to computer 28, which can control the flow through the secondline 23 by opening or closing the second valve 24. Fresh medium can beadmitted from the holding vessel 30 through first line 32. The flow ofthe medium can be controlled by computer 28 by opening or closing thefirst valve 34 and monitoring the flow using the first flow meter 36.The medium in the primary vessel 12 can be pressurized if it is a gas orconverted to a supercritical fluid. The medium becomes a supercriticalmedium when the temperature and pressure in the primary vessel 12 areadjusted to be greater than the critical temperature and criticalpressure, respectively, of the medium. The medium may also be nearcritical. Supercritical and near supercritical fluids may beadvantageous due to their high heat capacities. The pressure in theprimary vessel 12 may be monitored using the first pressure taps 38located high up in the primary vessel 12, above the surface 18 of thefluidized bed 16. The second pressure tap 39 is located low in thefluidized bed 16 slightly above the level of the distributor plate 14.The difference in pressure between pressure taps 38 and 39 can be usedto determine the mass of particles in the fluidized bed.

In some embodiments, the medium flows out of the primary vessel 12through third line 40. Particles entrained in the medium flowing out ofthe primary vessel 12 can be removed by the unit 42. The unit 42 may bea trap, a filter or a cyclone. All or part of the medium can becirculated back to the fluidized bed 16 by lines 43, 22 and 23 or can besent through the seventh line 44 to be purified in a separation unit,collected, released to atmosphere or sent to an atmospheric flare.Purified medium can be returned to the recycle line 43 through thefourth line 45. The flow of medium through the seventh line 44 can becontrolled by opening and closing the seventh valve 46 with the flowmeasured by the seventh flow meter 47. The fifth line 48 allows a purgefluid to enter the system 10 when the fourth valve 49 is opened. Therecirculating medium in the recycle line 43 can be cooled by the firstheat exchanger 50 and heated by the second heat exchanger 52. Heatingmay be needed prior to the start of the fabrication to raise thetemperature of the fluidized bed but below the sticky temperature atwhich particles adhere to each other. Cooling of the recirculatingmedium may be needed to remove heat supplied to the primary vessel 12 bythe energy beam. As can be appreciated there are different embodimentsof the lay-outs of the different streams and units such as heatexchangers, storage vessels, valves, lines and compressors of the systemthat would work. The layout in FIG. 1 is one example.

It can be appreciated that appropriate outputs from different componentsof the layout can be fed back to computer 28, and that the computer cantake appropriate actions according to well established control practicesby sending signals to open or close valves, or to control a heating unitor compressor or other such actions with other types of units. Thereforespecific connections between computer 28 and the other units in FIG. 1are very prolific but are not shown because these connections wouldrender the drawing difficult to read.

The temperature of the fluidized bed 16 can be monitored by thermocoupleor thermowell 54 connected to computer 28. An amount of heat can besupplied by the second heat exchanger 52 to heat the recirculatingmedium in the recycle line 43 and an amount of heat can be removed fromthe recirculating medium in the recycle line 43 by the first heatexchanger 50. The amount of heating and cooling is adjusted by computer28 to achieve a desired temperature in the fluidized bed. The fluidizedbed temperature can be adjusted further by using heat exchanger or aresistance heater 56 in the fluidized bed 16. A pressurized gas can beused as the medium in the system 10 and the method. Pressurizing the gasincreases the heat capacity of the gas and therefore allows for moreheat removal from the fluidized bed. A higher density gas can also makethe fluidized bed 16 more expandable when the fluidizing velocity isincreased. Especially if the particles have high density such as withmetal particles, then it may be advantageous to use a supercriticalmedium or a liquid as the medium.

In some embodiments, particles are held in the storage vessel 60 and canbe conveyed through the ninth line 61 and the tenth line 63 to theprimary vessel 12. The flow of particles can be controlled by computer28 by opening or closing valves 62 and 64. It can be appreciated thatthere is a wide range of equipment options available for conveyingparticles, including but not limited to pneumatic conveying and gravityassisted conveying. The particles in the storage vessel 60 can be heatedby the storage heating device 68 which comprises an electricalresistance heater, a heat exchanger or other heating device. The thirdtemperature monitoring device 69 is connected to the computer whichcontrols the amount of heat supplied to storage vessel 60 so that thetemperature of the particles in storage vessel 60 can be controlled. Thethird temperature monitoring device 69 may be a thermocouple, thermowellor any other suitable temperature monitoring device. The particles inthe storage vessel 60 can be in a fluidized bed, in which case, the linesupplying the fluidizing medium is not shown in the figure. Freshparticles fed through fourteenth line 67 are used to replenish theparticles in the storage vessel 60.

In addition, particles can be removed from the primary vessel 12 to thesalvage vessel 70 through eleventh line 71 by opening the eleventh valve72 and lining up the 3-way twelfth valve 74 to the salvage vessel 70through the twelfth line 73. Alternatively, the 3-way valve twelfthvalve 74 can be lined up to feed particles back to the storage vessel 60through thirteenth line 75. The particles in salvage vessel 70 can beheated by the fourth heating device 78 which comprises an electricalresistance heater, a heat exchanger or other heating device. The secondtemperature monitoring device 79, which may be a thermocouple,thermowell or suitable temperature monitoring device, is connected tothe computer which controls amount of heat supplied to the salvagevessel 70 by the fourth heating device 78 thus controlling thetemperature of the particles. The particle bed in the salvage vessel 70can be a fluidized bed. Particles can be returned to the primary vessel12 by conveying the particles through the eighth line 77 by opening theeighth valve 76.

Versions can include a moveable lift device 90 that may be controlled bythe computer 28 and can raise or lower the support frame 92 in theprimary vessel 12. Support frame 92 comprises a rigid frame and supportsa perforated build substrate 94. The support frame 92 is such that thefluidized bed 16 can easily pass through the opening of the frame 92.The frame 92 can be rectangular but as can be appreciated, many othergeometries are suitable, including extending mostly across the vessel12. The opening in the frame is such that the frame 92 does not blockoff portions of the build substrate 94 where fabrication is to start.The article 96 is fabricated on the build substrate 94. Build substrate94 has holes or pores, can be a perforated plate and can be composed ofmany types materials including the same or a similar type of material asthe particles. The size of the holes in the perforated plate 94 can beat least large enough to allow particles from the fluidized bed 16 topass through easily. Therefore the holes can be at least the averageparticle size d_(p) of the particles making up the fluidized particles.The number of holes can be sufficient in number and distribution suchthat holes are present at least in the vicinity where the fabrication isto occur. The holes may not be needed or desired in the area where nofabrication is to occur. In this embodiment the support frame ishorizontal but it can take on different geometries depending on thegeometry of the base of the article being fabricated. Hatch 98 in theprimary vessel 12 is closed during operation of the system 10 and can beopened to remove the completed article 96 or to access, maintain orclean the interior of the primary vessel 12 or lift mechanism 90.

In some embodiments, the cameras 106, 108 can monitor the level of thefluidized bed 16 and the state and temperature of the build. The outputof the cameras 106, 108 is used by the computer 28 to determine theheight of the surface 18 of the fluidized bed 16. The cameras 106, 108can also be used to determine the height of the article 96 above thebuild substrate 94 when it is lifted above the surface 18 of thefluidized bed 16. Visuals from the first cameras 106 can be used tomonitor for defects in the fabrication and infrared information from thesecond camera 108 can be used to detect temperature variations. It canbe appreciated that more than two cameras and cameras of different typeand resolution can be used.

To operate some embodiments of the system 10, valves 24, 41 and 53 areopened and pump or compressor 20 is turned on so as to circulate mediumthrough the primary vessel 12 and the lines 40, 43, 22 and 23. Particlesare heated to a desired temperature in the storage vessel 60. Valves 62and 64 are opened and the particles are conveyed into the primary vessel12 where it may be fluidized by the medium forced up through thedistributer plate 14. The amount of particles conveyed is sufficientsuch that when the particles are fluidized, the height of the surface 18of the fluidized bed 16 is slightly greater than the height of the topof the perforated build substrate 94. The mass of particles in thefluidized bed 16 can be determined from the difference in pressurebetween pressure taps 38 and 39. Cameras 106 and 108 can be used todetermine the height of the surface 18 of the fluidized bed 16 relativeto the perforated build substrate 94. For example the cameras 106 and108 can be used to determine when the top surface of the build substrate94 is level with the surface 18 of the fluidized bed 16. The buildsubstrate 94 has sufficient number and size of openings so that thefluidized bed 16 can envelope and go through the build substrate 94. Thecameras 106, 108 are also used to determine the location of the holes oropenings in perforated substrate 94. Lift 90 is then lowered by a smallamount such that the substrate 94 is immersed in the fluidized bed 16such that particles from the fluidized bed 16 covers the substrate 94.Laser 100 is turned on and beam 101 or beams 103 fuse the particles tosubstrate 94 in select locations. The target areas where the build-up ofthe article 96 starts are on the solid surface between the holes in theperforated surface so as not to block the openings. Computer 28manipulates optical components 102 to direct the aim of the beam orbeams 103 to selectively scan over and sinter particles at the targetareas of the substrate 94. The target areas lie within a horizontalcross section of the base of the three dimensional structure 96 to befabricated. As sintered particles builds up in the target area forming abottom portion of structure 96, the structure increases in height, lift90 lowers frame 92, substrate 94 and partially fabricated structure 96so that it remains immersed slightly below surface 18 of the fluidizedbed 16. The fused particles is cooled by up-flowing medium through theholes, and is positioned for additional particles to be transported bythe fluidized bed 16 through the article 96 to the build surface. As thearticle 96 builds up and is lowered, the computer 28 changes the aim ofbeam or beams 103 to new target areas that correspond to the nextcross-sectional slice of the article 96 to be fabricated. The beams 103then selectively sinters particles at the new target areas. There aremany methods of controlling the height of the structure being fabricatedrelative to the surface of the fluidized bed. One example is as follows.Computer 28 analyses images from cameras 106 and 108 to control thevertical position of lift 90 such that the surface of the article 96 isslightly below the surface of the fluidized bed 18. This can be done forexample by raising the top of the article 96 above the surface 18 of thefluidized bed to correlate the top with the surface 18 of the fluidizedbed with the cameras 106, 108, then lowering the article 96 by a smallamount into the fluidized bed 16.

The article 96 can be designed in such a way that it is largely porousand that the pores are continuous and connect to the openings in theperforated plate. The pores can also be interconnected and has an openporosity such that the pores are large enough relative to the particlesthat they can enter and be transported through the pores by the actionof the fluidizing medium.

The porosity of the article 96 is the fraction of void space in thearticle. The void space is defined by the plurality of pores. The volumefraction of solid material in the article 96 is the density p of thearticle 96 divided by the bulk density ρ_(s) of the solid material usedto manufacture the article 96. The porosity is one minus the volumefraction of solid material, as shown below.φ=1−ρ/ρ_(s)

In some embodiments, the porosity is greater than 50%. In someembodiments, the porosity is greater than 90%, 99%, 99.9% and 99.99%.The porosity affects various properties of the article 96 such as thespecific stiffness (stiffness to density ratio). For example, alightweight article 96 with high specific stiffness would also have highporosity. Near solid articles 96 have low porosity such as 50% to 90%.

There are several benefits of these embodiments including that thefluidized bed 16 transports particles to the build surface through thepores. The fluidized bed 16 regulates the temperature of the newly builtpart of the article 96. The fluidized bed 16 also provides buoyancy andso supports fragile members of the article 96 as long as the action ofthe fluidized bed 16 is not too vigorous. Additionally, the medium fromthe fluidized bed 16 can carry away volatiles from the sinteringprocess.

The method may manufacture articles 96 that have high stiffness todensity ratios. For lightweight porous material, the stiffness decreaseswith decreasing density p of the material. Young's modulus (E) is ameasure of stiffness. E_(s) is the Young's modulus of the solidmaterial. Lightweight materials such as foams and aerogels that arestochastic are known to decreases according to the below relationship.E/E _(s)˜(ρ/ρ_(s))³

As shown by the below relationship, the method may manufacture articles96 with a stiffness greater than stochastic materials.E/E _(s)>φ/ρ_(s))³

In some embodiments the stiffness of the article 96 follows the belowrelationship. For example, the specific stiffness, E/□, of a titaniumarticle 96 which has an E_(s) of 112.5 GPa and a bulk density ρ_(s) of4500 kg/m³ may be equal to or below 25×10⁶ m²/s².E/□≤E _(s)/ρ_(s)

In some embodiments, the stiffness of the article 96 follows the belowrelationship.E/□≥(ρ/ρ)² E _(s)/ρ_(s)

For example, the specific stiffness of a polystyrene article 96 whichhas a porosity φ of 0.9, an E_(s) of 3.2. GPa and a bulk density ρ_(s)of 1000 kg/m3 may be between 3.2×10⁶ m²/s² and 0.032×10⁶ m²/s².Polystyrene: 3.2×10⁶ m²/s² <E/ρ<0.032×10⁶ m²/s²

In addition to raising or lowering the lift 90, the surface 18 may beraised or lowered by many methods. Examples of such methods includeraising the surface 18 through bed expansion by increasing the flow offluidizing medium through the second line 23 by opening the second valve24. An additional example may be reducing the flow of fluidizing mediuminto the primary vessel 12 by closing the second valve 24 to lower thesurface 18. The computer 28 uses input on the level of the fluidized bedsurface 18 from the cameras 106, 108 or other level detection device 110in combination with medium flow data to determine the amount to open orclose the second valve 24. The fluidized bed 16 is achieved when thesuperficial velocity of the medium u fulfills the following:u≥u _(mf)where u_(mf) is the minimum fluidization superficial velocity. Thesuperficial velocity is also less than the bubbling velocity. A broadrange of small particle sizes can meet the above criterion but generallyare such that d_(p) is greater than about 10 micron. A wide variety ofparticle compositions, shapes and size distributions can be used and aresuitable to meet the velocity criterion above. Broadly speaking it isdesirable to have an expandable bed and by the term expandable it ismeant the ability to expand and contract the fluidized bed vertically byadjusting the velocity. Polymer particles with d_(p)<about 200 micronscan be suitable. For metals, particle density is generally high andsmaller particle sizes than 200 micron may be suitable. In addition todecreasing particle size, the fluidized bed 16 can be made moreexpandable by using a more dense fluidizing medium such as a pressurizedgas a supercritical medium or a liquid; by decreasing the particle sizeof the fluidized particles; or by increasing the viscosity of the fluid.

In some examples, based on the amount of particles removed from thefluidized bed 16 by being incorporated into a portion of the article 96,the computer 28 opens the tenth valve 64 and admits more particles fromthe storage vessel 60 to replenish the particles in fluidized bed 16 andmaintain the surface of the fluidized bed 18 above the article 96. Asthe fabrication process continues, the heat from the laser heats up thefluidizing medium. In order to maintain the desired temperature of thefluidized bed 16 as monitored by thermocouple 54, cooling device 50 isused to remove heat from the fluidizing medium. Heat exchanger 52 mayalso remove heat from the fluidizing medium.

Undesirable volatiles can be released by the sintering process. Thesevolatiles are carried away by the fluidizing fluid. As the volatilesbuild up in the fluid, they can be removed by purifying a portion of themedium through the seventh line 44 by opening the seventh valve 46 to apurification unit which can include a condenser or a distillation columnor suitable separation unit. Once purified, the medium can be returnedthrough the fourth line 45.

In one version, t process of adding material to the structure beingfabricated by selectively scanning and maintaining the top level of thestructure slightly below that of the fluidized bed continues untilfabrication of the final shape of the structure is complete.

In some embodiments, once the fabrication of the article 96 is complete,the laser 100 is turned away or off, and lift 90 is repositioned forremoval of the article 96 through hatch 98. To cool the part in acontrolled manner, the temperature of the fluidized bed 16 is lowered byusing cooling unit 50 and monitoring temperature with thermowell 54. Itmay be necessary to cool the article 96 in a controlled manner to avoidgeometric distortion. The valves 72, 74 are opened to remove particlesto salvage vessel 70 and lower the level 18 of the fluidized bed 16below the base of the fabricated part 96. The seventh valve 46 can beopened to vent out the system. The system can be purged by feeding purgemedium through the fifth line 48 by opening the fourth valve 49 andclosing the recycle-valve 53. Once the system is vented and cooled,valves 24 and 41 are closed to isolate the primary vessel 12, hatch 98is opened and the fabricated part 96 is removed. A new build substrate94 is attached to the support frame 92 and the hatch 98 is closed. Thesystem is then readied for the next fabrication by opening valves 24, 41and 53, closing the valves 49 and 46 and allowing fluidizing medium toenter the primary vessel 12 through the lines 22 and 23. Particles fromthe previous build in the salvage vessel 70 are conveyed to the primaryvessel 12 through lines 77 and 61 and by opening valves 76 and 62.

Turning now to examples of the fabricated article 96, FIG. 2 a-2 ddepict various stages of the fabrication process. FIG. 2 a depicts asection of the perforated build substrate 94 as seen from directlyabove. The direction of the medium velocity may be perpendicular to theperforated substrate 94 and can easily pass through the holes. The buildsubstrate 94 is attached to the support frame 92 and can be raised andlowered using lift device 90 controlled by computer 28. The perforatedplate 94 is lowered into the fluidized bed 16, so that a small portionof the fluidized bed 16 is above the perforated plate 94.

FIG. 2 b depicts the perforated surface as shown at an angle from abovewith the laser aimed at the surface of the substrate 94 adjacent to someholes. The direction of fluidizing medium flow is upwards through theholes. The arrow indicates the upward direction of the velocity throughone of the holes. The fluidized bed flows up through the holes in poroussubstrate 94. The size of the holes is sufficiently large so thatparticles can easily pass through the holes under the action of thefluidizing medium. The substrate 94 is lowered slightly below thefluidized bed surface 18, the action of the fluidized medium spreads theparticles over the submerged substrate 94 and laser beam or beams 103sinters particles to the substrate 94 in select locations. In thisembodiment the computer 28, directs the aim of the laser to scan overthe area of the perforated substrate 94 adjacent to the holes and buildsthe material in an upwards direction in such a way as to maintain apathways to the hole structure of the perforated substrate 94. In thisway a portion of the porous structure 96 is fabricated on the substrate94 as depicted in FIG. 2 c . As can be seen, the pores in the article 96fabricated by this method continue to allow the fluidizing medium andfluidized particles to pass through it. The medium in the fluidized bed16 flows up through the pores and transports particles through thepores. The article 96 is lowered slightly below the fluidized bedsurface 18, the action of the fluidized bed 16 spreads the particlesover the thus submerged article 96. The laser then sinters particles tothe top surface of the new structure in select locations to fabricatefurther portions of the structure. FIG. 2 d depicts a further portionthus fabricated. The process is continued until the structure iscompleted according to the model of the article in the computer.

Turning now to another example of the fabricated porous structure, FIG.3 a-3 g depict perspective views of various stages of the fabricationprocess of an open three dimensional honeycomb network structure. Thestructure is depicted very schematically as struts and nodes but maytake on many forms. FIG. 3 a depicts a perspective view of a portion ofthe perforated fabrication substrate 94. The process of fabrication issimilar to that described above in FIG. 2 a-2 d except that the porestructure is more open. In the example of FIG. 3 a-3 g the pores arecomposed of unit cells defined by struts connected at nodes in atetrahedral geometry. In FIG. 3 b the laser beam scans the target areasto form a portion of multiple predetermined unit cells. The article 96is started by building up vertical struts 130 from the surface of theperforated substrate 94 as shown in FIGS. 3 b and 3 c . The tops of thevertical struts form nodes 132 from which angled struts 134 are built upto connect at nodes 136 as depicted in FIGS. 3 d and 3 e . From nodes136, vertical struts 138 are built up as depicted in FIG. 3 f Thisprocess is continued to form a larger portion of the porous article 96as depicted in FIG. 3 g . The porous article 96 is interconnected andhas pores large enough that the medium can pass through the structureand continue to transport particles within the article 96. The processis further continued until the entire volume of the article 96 beingfabricated is filled in with the porous structure made up ofinterconnected struts.

Embodiments of the unit cells are composed of struts that can have across-sectional diameter as small as the size of a single particle orlarger. In some embodiments, the size of the particles are as small as10 microns in diameter. Particles smaller than 10 microns in diametermay begin cohering and may have difficulty smoothly fluidizing. As shownin FIG. 2 , the length of the struts, or any structure forming thearticle 96, may be as long as the article 96. In some embodiments, thelength of the each strut is greater than quadruple the particlediameter. In some embodiments, the width of the pores are greater thanabout 20 microns.

Embodiments of the unit cells are surface based, being composed ofsurfaces that can have a cross-sectional diameter as small as the sizeof a single particle or larger.

EXAMPLE EMBODIMENTS

Embodiments of the system 10 and the method of the present disclosureare included below but are not limited to those included below.

As can be appreciated it is not necessary for the porous structure to bebuilt of unit cells that have tetrahedral geometry. The unit cells canhave many different designs that give the structure a selected porosity.By varying the design of the unit cells, the orientation, shape and sizeof pores can be varied. For example, the unit cells can havedodecahedral, octahedral as well as many other types of shapes. The unitcells may take many forms both regular and irregular in size andorientation of struts. Irregular geometries may especially be needed todescribe the surfaces of the article. The orientation, size, shape andcomposition of the pores may be varied depending on various needs forthe properties of the final fabricated part.

The article 96 may comprise microlattice material that has repeatingcells defined by trusses (truss-lattice) or surfaces (surface based unitcell lattice). Microlattice materials can be lighter than air withdensities less than 1 kg/m³. Surface based unit cells are optimized overtruss based unit cells for higher specific stiffness. Examples ofsurface based unit cell lattices are shown in FIGS. 5 and 6 . Thearticle 96 may comprise triply periodic minimal surface (TPMS) latticesor any other repeating structure. The article 96 may comprises amicrolattice that is irregular. Such an irregular microlattice may havea Young's modulus greater than E_(s)/ρ_(s).

The structure of the article 96 may vary over its volume such as byincluding include different unit cells at different locations in thearticle 96. For example, the article 96 may transition from truss basedunits cell to surface based until cells over its length. Repeating unitcells are not necessary. The article may have a continually changingstructure, such as varying pore sizes and truss thicknesses.

As can be appreciated there can be different embodiments of the method.One embodiment is an iterative method. In this embodiment, the topsurface of the portion of the structure under fabrication is raised upby lift 90 above the fluidized bed surface 18, selectively heated aboveits sticky temperature or partially melted by the laser and thenimmersed below surface 18 in the bed where particles from the fluidizedbed impinge on the hot tacky surface and stick. The structure with theparticles sticking to the build surface can then be raised again abovethe fluidized bed and selectively heated by the laser, and sintered orfused to the build surface. The process is then continued with repeateddunking and irradiation of the structure so that successive layers ofparticles build up on the surface that is irradiated.

Another embodiment is a continuous method. In this mode, the structureis fabricated by a continuous process. The build surface is heldslightly below the surface of the fluidized bed. The build surface isirradiated to maintain a sufficiently high temperature so that it ispartially melted or has sufficient tackiness for particles that collidewith the surface to adhere and thus allow accretion to occur. As freshstructure builds up at the build surface, the structure is continuouslylowered or the surface of the fluidized bed continuously raised or bothso that the top of the build surface remains slightly below thefluidized bed surface. Multiple laser beams can be used to irradiatemultiple build surfaces simultaneously to trace out the pattern of theporous structure as the article is being lowered into the fluidized bed.

In another embodiment of the method, the particle bed is fluidizedintermittently. In this embodiment, the structure being fabricated issubmerged in the fluidized bed to a certain depth. The fluidization isthen stopped by reducing the velocity of the fluidization medium belowthe fluidization velocity by closing second valve 24. The depth to whichthe structure is submerged is such that when fluidization is stopped,the structure is covered by a thin layer of particles. Selective lasersintering can then take place on the quiescent layer of particlessitting on the surface of the structure so as to fabricate across-sectional slice of the structure. After sintering, the fluidizingmedium velocity is increased to above u_(mf) to re-fluidize the bed, thestructure is submerged once again and the process is repeated thusbuilding up successive layers of the structure.

Many types of particles are suitable. Examples of the type of particlesthat can be used can have the following properties:

-   -   The particles can be fused or sintered by an energy beam such as        a laser or an electron gun. Examples of such particles include        polymers, metals, ceramics, composites and mixtures thereof.        Examples of polymers include thermoplastic polymers such as but        not limited to polyamides, polypropylene and polyethylene as        well as thermosetting polymers. Examples of metals include but        are not limited to titanium and its alloys, aluminum, stainless        steel, cobalt chrome alloys, and other metals. The particles can        also be a mixture of one or more metal particles types with one        or more polymer particles types. The particles can also be        composed of particles comprising a mixed composition of one or        more metals and one or more polymers.    -   The particles can be fluidized. This means that for example the        particles are not too small that interparticle forces become        relatively large enough to make fluidization difficult.    -   The particle size distribution can be such that most particles        can easily pass through pores of the structure under        fabrication.

In addition to particles that are used for building the article,additives such as anti-oxidants for polymers can be mixed into thefluidized bed so that the additives are incorporated into the structure.

If a laser is used for the energy beam, then a CO₂ laser is one examplebecause of the power, availability and cost, but any type of laser thatcan sinter or fuse the particles is suitable. Lasers that areappropriate emit radiation with a wavelength that is sufficientlyabsorbed by the particles material to allow sintering to take place.Fabrication can take place in multiple locations on the surface bysplitting the laser beam into a multitude of beams each selectivelydirected at a target on the surface. Alternatively beams from multiplelasers can be used. The beam can be focused so as to concentrate powerin a small target area. In certain cases where the relative sinteringpower of the laser is strong, the beam can be passed through a mask sothat a large cross-sectional area can be fabricated at the same time.The laser can be directed by different methods including by oscillatingthe beam aim with an oscillating mirror. A pulsing beam or a continuousbeam can be used.

Many different types of fluids can be used that are suitable for bothfluidization and heat removal. These fluids can be gases, liquids orsupercritical fluids. Non-limiting examples of gases are air, nitrogen,carbon dioxide, helium, neon, argon. The fluidizing gas can bepressurized. It may be advantageous for heat removal or fluidizationquality to use a pressurized gas. Instead of a gas as fluidizing mediumit may be advantageous to use a supercritical medium such as but notlimited to supercritical propane, ethane or carbon dioxide. Thefluidizing medium can be either inert or reactive to the structure. Forsome applications a reactive gas such as methane or hydrogen can be usedwhen the particles is a metal. For particles with high density such asmetals it can be advantageous to use a fluidizing medium with higherdensity than a gas such as a supercritical medium or liquid.

In one embodiment, the fluidizing medium is not recycled. Gas from theholding vessel 30 or air is used for fluidization in in the primaryvessel 12 and collected or vented to the atmosphere through lines 40 and44.

An alternative example of the layout in FIG. 1 has different particletypes in multiple vessels such as storage vessel 60 from which it ispossible to feed different particles types to the fluidized bed in theprimary vessel 12. By feeding different types of particles duringdifferent phases of the fabrication, it is possible to change thecomposition of the particles of the fluidized bed to a differentparticles type. Since the composition of the fabricated structure isdependent on the composition of the particles in the bed, as thefluidized bed particles composition changes, the composition ofparticles being sintered changes and the composition of the fabricatedstructure changes. If the composition of particles in the fluidized bedis changed during fabrication then the article 96 will have a gradientof compositions. In this way an article can be fabricated with varyingproperties at different locations in its volume. The composition can bechanged either gradually or rapidly, in some versions. If thecomposition of the fluidized bed is changed rapidly, then the materialcomposition of the article may change over the length scale of a singleparticle. If the composition of the fluidized bed is changed gradually,then the length scale of change in the article would be larger, forexample, 1 cm. This type of article can be useful for a number ofdifferent applications including mechanical and optical. For example,the material of the article 96 may be varied between a stiff materials,such as metals, to a rubbery material, such as certain polymers, indifferent locations of the article thus giving it special mechanicalproperties. Examples of polymers that are rubbery include elastomerslike polyisoprene, polybutadiene, styrene-butadiene rubber (copolymer ofstyrene and butadiene), EPM (ethylene propylene rubber, a copolymer ofethylene and propylene), EPDM rubber (ethylene propylene diene rubber, aterpolymer of ethylene, propylene and a diene-component), and ethylenevinyl acetate (EVA).

It can be appreciated that the equipment that services an FBAM unit,including particles holding and medium holding vessels,instrumentations, valves, heat exchangers, units for purification andpumps (or compressors) in the layout in FIG. 1 can add considerable costper article fabricated. In an embodiment, much of this equipment isshared over multiple FBAM units such as FBAM unit 10 depicted in FIG. 1each comprising a vessel much like the primary vessel 12 fitted with adistributor plate, laser, lift, cameras and individual temperaturecontrol. Each FBAM unit is connected to a computer or multiple computersthat control the additive manufacturing and fluidized bed process ineach vessel. The multiple FBAM units can be configured in an array andserviced by a shared cooler, compressor, and particles handling system.An example of an array 140 of four FBAM units 141, 142, 143 and 144 isdepicted in FIG. 4 . As can be appreciated, many more than four FBAMunits can be configured in a vast array including hundreds, thousands ormore FBAM units. For each additional FBAM unit in the array, theparticles handling, storage, compressor, purification and heating unitscan be sized accordingly to be able to handle the additional particlesand medium requirements. Note that in FIG. 4 , the valves in the linesare not shown but are appropriately placed, particularly so that thefluidization in each FBAM unit can be controlled separately, andparticles can be added and removed to each FBAM unit independently. EachFBAM unit can be individually isolated from the medium flow by closingvalves similarly placed to valves 24 and 41 in FIG. 1 on each FBAM unit141, 142, 143 and 144.

The sintering or fusing process in each FBAM unit generates volatilesthat would build up in the recirculating medium stream. In the exampledepicted in FIG. 4 , a purification unit partially separates volatilesfrom stream 44 in separation unit 150. Volatiles separated by unit 150are removed through line 152. Purified medium is returned torecirculation recycle line 43 through the fourth line 45.

OTHER EMBODIMENTS

It can be appreciated from the foregoing that while certain examples ofthis disclosure have been depicted and described, they do not limit theembodiments and modifications that can be made without departing fromthe spirit and scope of this disclosure. Other embodiments can includeone or more of the following versions.

A method of fabrication comprising:

fluidizing particles with a medium to form a fluidized bed having asurface 18,

additively manufacturing an article comprising the particles, thearticle having an open porosity,

forming a plurality of pores in the article that define fluid pathsthrough the article, and

flowing the particles and the medium through the fluid paths while thearticle and the fluid paths are being formed.

The method of any of these embodiments wherein additively manufacturingthe article further comprises additively manufacturing the article witha non-stochastic structure.

The method of any of these embodiments further comprising simultaneouslyflowing the particles and the medium through all of the pores in thearticle.

The method of any of these embodiments wherein additively manufacturingfurther comprises flowing produced volatiles away from the article.

The method of any of these embodiments further comprising cooling thefluidized bed and the article while additively manufacturing thearticle.

The method of any of these embodiments further comprising:

heating the particles at a target on the article, and

stabilizing the article by flowing the particles and the medium adjacentthe target.

The method of any of these embodiments wherein forming the plurality ofpores comprises forming interconnected pores.

The method of any of these embodiments wherein additively manufacturingthe article further comprises additively manufacturing a plurality ofstruts converging at nodes comprising the article, such that the strutsand nodes define the pores.

The method of any of these embodiments further comprising filling someof the pores with the particles to further form the struts and nodes.

The method of any of these embodiments further comprising:

providing the article with a volume, and

filling a majority of the volume with the particles and the medium.

The method of any of these embodiments further comprising substantiallyfilling an entirety of the volume with the particles and the medium.

The method of any of these embodiments wherein the plurality of porescomprises a majority of the volume.

The method of any of these embodiments wherein flowing the particles andthe medium through the fluid paths comprises pumping the particles andmedium through the fluid paths.

The method of any of these embodiments wherein flowing the particles andthe medium through the fluid paths comprises enveloping the articlewithin the fluidized bed by moving the article.

The method of any of these embodiments wherein flowing the particles andthe medium through the fluid paths comprises enveloping the articlewithin the fluidized bed by changing the height of the fluidized bed.

The method of any of these embodiments wherein flowing the particles andthe medium through the fluid paths comprises changing a pressure of themedium to change a level of the surface.

The method of any of these embodiments further comprising:

additively manufacturing a top on the article, and further comprising:

leveling the top of the article with the surface 18 of the fluidized bedsuch that the fluidized bed envelopes the top and the top is adjacentthe surface 18 of the fluidized bed;

continuously lowering the article into the fluidized bed; or

continuously raising the surface of the fluidized bed.

The method of any of these embodiments further comprising:

supporting the article on a substrate defining a plurality of channelsextending through the substrate, and

flowing particles and the medium through all of the channels in thesubstrate.

The method of any of these embodiments further comprising binding thearticle to the substrate.

The method of any of these embodiments wherein the substrate is a mesh.

The method of any of these embodiments further comprising:

defining the particles with an average diameter dp, and defining thechannels with a minimum diameter of at least about twice the averagediameter dp of the particles.

The method of any of these embodiments wherein the channels have varyingdiameters.

The method of any of these embodiments wherein the fluid paths have aminimum diameter of at least about twice the average diameter dp of theparticles.

The method of any of these embodiments 8 further comprising forming thesubstrate from a same material as the particles.

The method of any of these embodiments further comprising pressurizingthe medium to increase a heat capacity of the medium and therebyincreasing a rate of heat dissipation.

The method of any of these embodiments further comprising:

providing the particles as first particles and second particles whereinthe first particles and the second particles comprise differentmaterials,

temporally varying a ratio of the first particles to the secondparticles in the fluidized bed,

additively manufacturing the article having a spatially varied materialcomposition of the first particles and the second particles.

The method of any of these embodiments wherein the particles have anaverage outer dimension of at least about 10 microns and not greaterthan about 1 mm.

The method of any of these embodiments wherein the particles comprise ametal.

The method of any of these embodiments wherein the particles comprise apolymer.

The method of any of these embodiments wherein the polymer comprises amaterial selected from the group consisting of polyamide 6, polyamide12, polypropylene, polyethylene, polyethylene terephthalate,polybutylene terephthalate, polyoxymethylene, polystyrene, poly(methylmethacrylate) or any combination thereof.

The method of any of these embodiments wherein additively manufacturingfurther comprises visually monitoring the additive manufacturing withina transparent zone of the fluidized bed, the transparent zone extendinga distance D below the surface 18 of the fluidized bed.

The method of any of these embodiments further comprising increasing thedistance D of the transparent zone by increasing a velocity of themedium of the fluidized bed.

The method of any of these embodiments further comprising maintainingthe particles within a vessel, and continuously circulating the mediuminto the vessel, through the particles and out of the vessel.

The method of any of these embodiments further comprising sealing themedium and the particles within a closed system.

The method of any of these embodiments further comprising focusing aplurality of energy beams onto a target on the article to concentratethe power of the energy beams.

The method of any of these embodiments wherein the energy beam is alaser beam.

The method of any of these embodiments wherein the energy beam is anelectron beam.

The method of any of these embodiments where the medium is selected fromthe group consisting of air, nitrogen, carbon dioxide and an inert gas,or any combination thereof.

The method of any of these embodiments wherein the medium is asupercritical fluid.

The method of any of these embodiments wherein fluidizing the particleswith the medium to form the fluidized bed further comprises:

conveying the particles into a primary vessel 12 having a top and abottom and defining a chamber and conveying the particles above adistributor plate 14 extending horizontally across the chamber near thebottom of the primary vessel 12, the distributor plate having aplurality of holes, and

pumping the medium upward through the holes in the distributor plate 14such that a superficial velocity of the medium is greater than a minimumfluidization superficial velocity u_(mf).

The method of any of these embodiments further comprising:

additively manufacturing a first horizontal cross section of the articlewithin the fluidized bed and adjacent the surface 18 of the fluidizedbed, and

additively manufacturing one or more additional horizontal crosssections on top of the first horizontal cross section wherein all of thehorizontal cross sections define the article.

The method of any of these embodiments wherein additively manufacturingcomprises sintering the particles, with the particles having a sinteringwindow defined as a temperature range from a crystallization temperatureto a melting temperature.

The method of any of these embodiments further comprising depositingparticles on the article by reducing the velocity of the medium to belowthe minimum fluidization superficial velocity u_(mf).

The method of any of these embodiments further comprising:

wherein the article comprises a top and a bottom, and

heating the top to maintain the temperature of the top in the sinteringwindow and cooling the bottom to below the sintering window to create atemperature gradient within the article.

The method of any of these embodiments wherein sintering the particlescomprises emitting an energy beam at a target on the article.

The method of any of these embodiments wherein additively manufacturingthe article further comprises:

additively manufacturing the article comprising a microlattice that isirregular.

The method of any of these embodiments wherein additively manufacturingthe article further comprises:

additively manufacturing the article comprising a material that has aYoung's modulus E_(s) and a density ρ_(s),

additively manufacturing the article with a Young's modulus E and adensity p that follow the relationship: E/□≥E_(s)/ρ_(s).

The method of any of these embodiments wherein additively manufacturingthe article further comprises:

additively manufacturing the article comprising a material that has aYoung's modulus E_(s) and a density ρ_(s),

additively manufacturing the article with a Young's modulus E and adensity p that follow the relationship: E/□≥(ρ/ρ_(s))² E_(s)/ρ_(s).

The method of any of these embodiments wherein additively manufacturingan article further comprises additively manufacturing the article thearticle a Young's modulus E and a p that follow the relationship:E/□≤E_(s)/ρ_(s).

The method of any of these embodiments wherein additively manufacturingthe article further comprises:

additively manufacturing the article with a porosity φ of at least about50%.

The method of any of these embodiments wherein additively manufacturingthe article further comprises:

additively manufacturing the article with a porosity φ of at least about60%.

The method of any of these embodiments wherein additively manufacturingthe article further comprises:

additively manufacturing the article with a porosity φ of at least about70%.

The method of any of these embodiments wherein additively manufacturingthe article further comprises:

additively manufacturing the article with a porosity φ of at least about80%.

The method of any of these embodiments wherein additively manufacturingthe article further comprises:

additively manufacturing the article with a porosity φ of at least about90%.

The method of any of these embodiments wherein additively manufacturingthe article further comprises:

additively manufacturing the article with a porosity φ of at least about95%.

The method of any of these embodiments wherein additively manufacturingthe article further comprises:

additively manufacturing the article with a porosity φ of at least about99%.

The method of any of these embodiments wherein additively manufacturingthe article further comprises:

additively manufacturing the article with a porosity φ of at least about99.9%.

The method of any of these embodiments wherein additively manufacturingthe article further comprises:

additively manufacturing the article with a porosity φ of at least about99.99%.

The method of any of these embodiments wherein additively manufacturingthe article further comprises:

additively manufacturing the article with a porosity φ of not greaterthan 50%.

The method of any of these embodiments wherein additively manufacturingthe article further comprises:

additively manufacturing the article with a porosity φ of not greaterthan 60%.

The method of any of these embodiments wherein additively manufacturingthe article further comprises:

additively manufacturing the article with a porosity φ of not greaterthan 70%.

The method of any of these embodiments wherein additively manufacturingthe article further comprises:

additively manufacturing the article with a porosity φ of not greaterthan 80%.

The method of any of these embodiments wherein additively manufacturingthe article further comprises:

additively manufacturing the article with a porosity φ of not greaterthan 90%.

The method of any of these embodiments wherein additively manufacturingthe article further comprises:

additively manufacturing the article with a porosity φ of not greaterthan 95%.

The method of any of these embodiments wherein additively manufacturingthe article further comprises:

additively manufacturing the article with a porosity φ of not greaterthan 99%.

The method of any of these embodiments wherein additively manufacturingthe article further comprises:

additively manufacturing the article with a porosity φ of not greaterthan 99.9%.

The method of any of these embodiments wherein additively manufacturingthe article further comprises:

additively manufacturing the article with a porosity φ of not greaterthan 99.99%.

The method of any of these embodiments wherein the pores have a minimumdiameter of at least about 20 microns.

The method of any of these embodiments wherein additively manufacturingthe article further comprises:

additively manufacturing the article comprising particles with averageouter dimension d_(p), and

additively manufacturing the struts having a length that is quadruplethe average outer dimension d_(p).

The method of any of these embodiments wherein additively manufacturingthe article further comprises:

additively manufacturing the article comprising a microlattice.

The method of any of these embodiments wherein additively manufacturingthe article further comprises:

additively manufacturing the article comprising repeating unit cellsthat are surface based.

The method of any of these embodiments wherein additively manufacturingthe article further comprises:

additively manufacturing the article comprising repeating unit cellsthat are surface based.

The method of any of these embodiments wherein additively manufacturingthe article further comprises:

additively manufacturing the article comprising triply periodic minimalsurface lattices.

The method of any of these embodiments wherein additively manufacturingthe article further comprises:

additively manufacturing the article comprising repeating unit cellsthat are truss based.

A system 10 for fabricating an article, the system comprising:

a primary vessel 12 being impermeable to fluids and insulated and havinga vertical axis Z and a horizontal axis X perpendicular to the verticalaxis Z and a top and bottom being vertically spaced apart,

the primary vessel 12 including a window 104 at the top,

the primary vessel 12 defining a chamber 13 and having a hatch 98 whichcan be opened to provide access to the chamber,

a distributor plate 14 disposed in the primary vessel 12 and extendinghorizontally across the chamber, the distributor plate having aplurality of holes, and

a fluidized bed disposed in the chamber and having a surface 18 abovethe distributor plate 14, the fluidized bed being expandable and havingparticles, the particles having an average outer dimension d_(p) that isat least about 10 microns and not greater than about 1 mm, the fluidizedbed having a medium with a superficial velocity u that is greater than aminimum fluidizing superficial velocity u_(mf) of the particles.

The system 10 of any of these embodiments wherein the fluidized bed hasa transparent zone that is at least partially transparent and whichextends a distance D below the surface 18 and the article is formed inthe transparent zone.

The system 10 of any of these embodiments further comprising:

a lift device 90 in the chamber 13 and attached to the primary vessel 12being capable of moving vertically, and

a frame attached 92 to the lift device 90 and being vertically movableby the lift device 90 and extending at least partially around theperimeter of the primary vessel 12 and having openings 93.

The system 10 of any of these embodiments further comprising:

a substrate 94 disposed in the chamber and defining channels that are ofvarying size and that are larger than average outer dimension d_(p) ofthe particles such that the particles can flow through the channels,holes and openings.

The system 10 of any of these embodiments further comprising:

a first camera 106 mounted adjacent the primary vessel 12 and directedtoward the surface 18 for visually monitoring fabrication of thearticle.

The system 10 of any of these embodiments further comprising:

a second camera 108 of an infrared type mounted adjacent the primaryvessel 12.

The system 10 of any of these embodiments further comprising an energybeam source 100.

The system 10 of any of these embodiments further comprising, for theenergy beam source, optical components 102 comprising at least one prismand at least one mirror assembly comprising at least one mirror coupledto at least one galvanometer.

The system 10 of any of these embodiments further comprising:

a first heat exchanger 56 coupled to the primary vessel 12, and

a first temperature monitoring device 54 disposed in the chamber.

The system 10 of any of these embodiments further comprising a firstpressure tap 38 coupled to the primary vessel 12 toward the top andabove the fluidized bed and a second pressure tap 39 coupled to theprimary vessel 12 above the distributor plate 14.

The system 10 of any of these embodiments further comprising a recycleline 43 coupled to the first heat exchanger 50 and a second heatexchanger 52.

The system 10 of any of these embodiments further comprising a holdingvessel 30 containing the medium and being fluidly connected to thechamber.

The system 10 of any of these embodiments further comprising

a salvage vessel 70 containing some of the particles and the medium andbeing fluidly connected to the chamber such that particles and themedium and can be conveyed from the salvage vessel 70 to the chamber,

a third heat exchanger 78 coupled to and extending into the salvagevessel 70,

a second temperature monitoring device 79 coupled to and extending intothe salvage vessel 70,

a storage vessel 60 containing some of the particles and being fluidlyconnected to the chamber such that the particles and the medium can flowfrom the storage vessel 60 to the chamber,

a storage heat exchanger 68 coupled to and extending into the storagevessel 60, and

a third temperature monitoring device 69 coupled to and extending intothe storage vessel 60.

The system 10 of any of these embodiments further comprising a recycleline 43 coupled to the first heat exchanger 50 and the second heatexchanger 52, and a pump 20 fluidly connected to the chamber 13.

The system 10 of any of these embodiments further comprising a computer28 configured to receive information regarding temperature, pressure andvisuals for the system, and to control the pump 20 based on theinformation received.

What is claimed is:
 1. A method of fabrication comprising: fluidizingparticles with a medium to form a fluidized bed having a surface,additively manufacturing an article comprising the particles, thearticle having an open porosity, forming a plurality of pores in thearticle that define fluid paths through the article, flowing theparticles and the medium through the fluid paths while the article andthe fluid paths are being formed; providing the particles as firstparticles and second particles wherein the first particles and thesecond particles comprise different materials, temporally varying aratio of the first particles to the second particles in the fluidizedbed, and additively manufacturing the article having a spatially variedmaterial composition of the first particles and the second particles. 2.The method of claim 1, further comprising simultaneously flowing theparticles and the medium through the pores in the article.
 3. The methodof claim 1, wherein forming the plurality of pores comprises forminginterconnected pores.
 4. The method of claim 1, further comprisingproviding the article with a volume, such that the plurality of porescomprises a majority of the volume.
 5. The method of claim 1, furthercomprising: additively manufacturing a top on the article, and furthercomprising: leveling the top of the article with the surface of thefluidized bed such that the fluidized bed envelopes the top and the topis adjacent the surface of the fluidized bed; continuously lowering thearticle into the fluidized bed; or continuously raising the surface ofthe fluidized bed.
 6. The method of claim 1, further comprising:supporting the article on a substrate defining a plurality of channelsextending through the substrate, and flowing particles and the mediumthrough the channels in the substrate.
 7. The method of claim 6, furthercomprising binding the article to the substrate.
 8. The method of claim6, further comprising: defining the particles with an average diameterd_(p), and defining the channels with a minimum diameter of at leastabout twice the average diameter d_(p) of the particles.
 9. The methodof claim 8, wherein the channels have varying diameters.
 10. The methodof claim 8, wherein the fluid paths have a minimum diameter of at leastabout twice the average diameter d_(p) of the particles.
 11. The methodof claim 1, wherein additively manufacturing further comprises visuallymonitoring the additive manufacturing within a transparent zone of thefluidized bed, the transparent zone extending a distance D below thesurface of the fluidized bed.
 12. The method of claim 11, furthercomprising increasing the distance D of the transparent zone byincreasing a velocity of the medium of the fluidized bed.
 13. The methodof claim 1, wherein the medium is a supercritical fluid.
 14. The methodof claim 1, wherein fluidizing the particles with the medium to form thefluidized bed further comprises: conveying the particles into a primaryvessel having a top and a bottom and defining a chamber and conveyingthe particles above a distributor plate extending substantially entirelyacross the primary vessel adjacent the bottom of the primary vessel, thedistributor plate having a plurality of holes, and pumping the mediumthrough the holes in the distributor plate such that a superficialvelocity of the medium is greater than a minimum fluidizationsuperficial velocity u_(mf).
 15. The method of claim 1, wherein thefluidized bed is expandable and comprises particles, the particles havean average outer dimension d_(p) that is at least about 10 microns andnot greater than about 1 mm.
 16. The method of claim 1, furthercomprising: visually monitoring fabrication of the article with a firstcamera.
 17. The method of claim 1, further comprising: containing atleast some of the particles and the medium with a salvage vessel;fluidly connecting the salvage vessel to the chamber and conveying theparticles and the medium from the salvage vessel to the chamber; atemperature monitoring device coupled to and extending into the salvagevessel, fluidically connecting a storage vessel to the chamber andflowing at least some of the particles and the medium from the storagevessel to the chamber.
 18. The method of claim 17, further comprising: aheat exchanger coupled to and extending into the salvage vessel; astorage heat exchanger coupled to and extending into the storage vessel,and another temperature monitoring device coupled to and extending intothe storage vessel.
 19. The method of claim 1, wherein the mediumcomprises a gas.
 20. A method of fabrication comprising: fluidizingparticles with a medium to form a fluidized bed having a surface,additively manufacturing an article comprising the particles, thearticle having an open porosity, forming a plurality of pores in thearticle that define fluid paths through the article; flowing theparticles and the medium through the fluid paths while the article andthe fluid paths are being formed; wherein additively manufacturingfurther comprises visually monitoring the additive manufacturing withina transparent zone of the fluidized bed, the transparent zone extendinga distance D below the surface of the fluidized bed; and increasing thedistance D of the transparent zone by increasing a velocity of themedium of the fluidized bed.
 21. A method of fabrication comprising:fluidizing particles with a medium to form a fluidized bed having asurface, additively manufacturing an article comprising the particles,the article having an open porosity, forming a plurality of pores in thearticle that define fluid paths through the article; flowing theparticles and the medium through the fluid paths while the article andthe fluid paths are being formed; containing at least some of theparticles and the medium with a salvage vessel; fluidly connecting thesalvage vessel to the chamber and conveying the particles and the mediumfrom the salvage vessel to the chamber; a temperature monitoring devicecoupled to and extending into the salvage vessel; and fluidly connectinga storage vessel to the chamber and flowing at least some of theparticles and the medium from the storage vessel to the chamber.